<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="js/vue-2.6.10.js"></script>
</head>
<body>
<div id="app">

    <h2>
        {{new Date(birthday).getFullYear()}}-{{new Date(birthday).getMonth()+1}}-{{new Date(birthday).getDay()}}
    </h2>

    <hr>
    <h1>
        {{birth}}
    </h1>
</div>
<script type="text/javascript">
    new Vue({
        el: "#app",
        data: {
            birthday:1212121212
        },
        computed:{
            birth(){
                const  date = new Date(this.birthday);
                return date.getFullYear() + "-" + (date.getMonth()+1) + "-" + date.getDay();
            }
        }
    });
</script>
</body>
</html>